This invention generally relates to protection circuitry. The invention more specifically relates to over-voltage protection circuits.
There may be various types of protective circuits such as short-circuit protection, over-voltage protection, under-voltage lockout and thermal shutdown. One of the most damaging faults that may occur in voltage regulator circuits may be output over-voltage. Output over-voltage from a voltage regulator circuit may result in the destruction of all or many ICs (integrated circuits) that are being powered by a failing regulator. In the case of a PC (personal computer), very expensive components, such as an MPU (microprocessor unit) may be destroyed. Thus, the cost of failure may be far in excess of the cost of the voltage regulator. In an effort to protect expensive components from destruction, over-voltage protection circuits may be used.
One form of over-voltage protection is the crowbar circuit. In a crowbar circuit when an over-voltage condition is detected, a heavy-duty switch, for example, an SCR (silicon controlled rectifier) is closed so as to effectively short-circuit the output voltage. The resultant heavy current is typically intended to blow a fuse and thus, or otherwise, shut down the over-voltage before there is time to permanently damage expensive components downstream. Crowbar circuits typically use a latching mechanism or a hysteresis circuit to prevent oscillation and/or marginal switching. Crowbar circuits provide good over-voltage protection but they are relatively expensive. As such, crowbar circuits are typically employed in more expensive server computers where the relatively higher costs are readily justified.
Inexpensive and moderately priced PCs are commonly built without a crowbar protection circuit for reasons of economy. Other, cheaper but less effective, forms of over-voltage protection circuit are found in the switching power supply regulators of less expensive computers. The cheaper forms of protection circuit have poorer performance and/or reliability.
The invention provides for circuit protection including over-voltage protection.
According to an aspect of the invention, a circuit is used for providing a regulated voltage. The circuit comprises an upper transistor connected to an input voltage from a voltage source, a lower transistor connected to the upper transistor, a pulse width modulator for generating control signals for the upper and lower transistors and a voltage protection circuit. The voltage protection circuit comprises an over-voltage detector circuit powered by the regulated voltage. The voltage protection circuit can cause the lower transistor to draw sufficient current from the voltage source such that an over-voltage condition becomes abated.
According to another aspect of the invention, a circuit for protecting against over-voltage is provided. The circuit comprises an over-voltage detector powered by a regulated voltage operable to generate an over-voltage detected signal, an amplifier powered by the regulated voltage operable to generate a trigger signal in response to the over-voltage detected signal, and a thyristor adapted to clamp the regulated voltage in response to the trigger signal.
Other aspects of the invention are possible, some are described below.